NAME

       diag - get diagonal part of a matrix

DESCRIPTION

       This function get the diagonal part of a matrix.

               csr<Float> a;
               dia<Float> d = diag(a);

TODO

       Build a csr matrix from a diagonal one or from a vector:

               dia<Float> d;
               csr<Float> a = diag(d);
               vec<Float> u;
               csr<Float> b = diag(u);

IMPLEMENTATION

       template<class T, class M>
       dia<T,M> diag (const csr<T,M>& a)